I. Photolithography and Photoresists in Semiconductor Manufacturing
The size of integrated circuits is at least partially limited by the ability of the manufacturing methods to be carried out on a small scale. Many steps in the manufacture of integrated circuits can involve photolithography, in which patterns of features are produced in photoresist materials. Photoresist materials are sensitive to electromagnetic radiation, and upon development of the photoresist layer, portions of the photoresist layer are removed, by a process termed photolysis, revealing the underlying semiconductor material. Subsequent exposure of the underlying semiconductor material to etchants can result in the removal of the semiconductor material only where the photoresist layer had been photolyzed. Subsequently, the remaining regions of photoresist material are removed, leaving the un-etched areas of the semiconductor exposed for further processing.
II. Limitations of Feature Size Photolithography
The minimum size of a feature that can be manufactured on a semiconductor wafer is called a critical dimension, which can be limited by photolithography processes. For certain photolithography methods, it is desirable for the incident beam of electromagnetic radiation to penetrate into the photoresist layer in a direction perpendicular to the photoresist layer and the semiconductor wafer. Vertical orientation can provide desirable high resolution of photolithography, thereby minimizing critical dimensions. However, resolution of a photolithography step can be limited, for example, by diffraction of electromagnetic radiation by the edges of the mask and reflection of electromagnetic radiation by underlying layers. Collectively, there effects widen the area of photoresist exposed to electromagnetic radiation, a process termed herein "beam spreading." Nitride and oxynitride layers can amplify the problems inherent in photolithography, and thereby can limit the size of device features.
III. Diffraction, Reflection and Interference Effects in Photolithography
A. Diffraction
Diffraction of electromagnetic radiation by the edge of a mask ("edge effect diffraction") can displace the direction of incident electromagnetic radiation toward more lateral areas of photoresist which underlie the mask. Lateral displacement of the beam can expose undesired areas of photoresist, including areas of photoresist under the mask itself. Angular displacement of electromagnetic radiation is dependent on the wavelength of the radiation, with longer wavelengths being deflected by a larger angle than shorter wavelengths. This has led to the use of higher-energy, shorter wavelengths in photolithography.
Additionally, after angular displacement of a beam of electromagnetic radiation, the total lateral distance away from its intended path that a beam can travel is dependent on the thickness of the layers through which it passes. A thicker layer permits a greater lateral displacement of the beam. Therefore, another approach to decreasing the effect of diffraction is to decrease the thickness of the photoresist film. By decreasing the thickness of the film, there can be less opportunity for diffracted electromagnetic radiation to undercut the photoresist. However, as the photoresist film thickness is reduced, there can be increased variation in thickness of the photoresist layer, leading to errors in transfer of an image to the photoresist. Moreover, as the layer of photoresist becomes thinner, the transparency of the photoresist layer increases, thereby increasing reflection of electromagnetic radiation by underlying surfaces.
B. Interface Reflection
Interfaces between layers of materials can reflect incident electromagnetic radiation. Interfaces relevant to semiconductor manufacturing include, by way of example only, interfaces between silicon oxides and silicon. When a source-drain stack is manufactured using layers of oxide, stoichiometric nitride or oxynitride, and photoresist, the electromagnetic radiation can pass through the photoresist layer, the nitride or oxynitride, and the oxide, and can be reflected back upwards through the stack. Lateral reflection can cause absorption of electromagnetic radiation by photoresist underneath the mask edge, undercutting the mask edge and resulting in additional inaccuracies in the transfer of the mask image to the photoresist.
Where barrier or polish-stop layers underlie photoresists, they can add to the critical dimension problem. Polish-stop and barrier layers serve several purposes in manufacturing semiconductor devices. Polish-stop layers can be used when it is desired to provide a surface below which an etching or chemical mechanical polishing step will not remove substantial amounts of material. Barrier layers are commonly used in another type of isolation, termed the local oxidation of silicon (the "LOCOS") method. Barrier layers typically retard diffusion of contaminants into semiconductor structures.
Silicon nitride and silicon oxynitride are examples of materials commonly used to form barrier or polish-stop layers in photolithography processes. As used herein, the term "barrier layer" can refer to films that act either as diffusion barriers or as polish-stop layers. The chemical formula of silicon nitride is: Si.sub.3 N.sub.4, and the formula for silicon oxynitride is: Si.sub.3 N.sub.4 O.sub.x, where x can vary from less than about 1 to about 3. Silicon nitride films can be made using chemical vapor deposition (CVD), wherein precursors, by way of example only, SiH.sub.4 and NH.sub.4 are introduced into a deposition apparatus. A source of energy dissociates the precursors into reactive intermediates, which then can combine on the wafer surface to form the layer of nitride. Oxynitride films can be made by introducing N.sub.2 O or NO into the reaction chamber. A desirable property of these materials for use as polish-stop layers include high mechanical strength, and a desirable property of these materials for use as barrier layers include high resistance to diffusion of contaminant molecules. These desirable properties of nitride and oxynitride are the greatest for stoichiometric films, that is, films in which the ratio of silicon to nitrogen is 3:4. However, conventional, stoichiometric nitride and oxynitride layers can provide problems in photolithography, including reflection and standing wave effects which make the manufacture of small, reproducible semiconductor device features difficult.
As manufacturing processes become more miniaturized, barrier and polish-stop layers become thinner. However, it is desirable to maintain desired mechanical and chemical properties of barrier and polish-stop layers. Low Pressure Chemical Vapor Deposited (LPCVD) silicon nitride layers can be made with these desirable qualities because nitride layers can be made which are thin and stoichiometric, thus comprising Si.sub.3 N.sub.4. However, stoichiometric nitride layers can be transparent. In patterning using monochromatic electromagnetic radiation, transparency poses a limitation as the critical dimensions become smaller. Additionally, silicon oxide layers underlying the nitride layers also can be transparent. In contrast, interfaces between oxide and silicon substrate layers can reflect electromagnetic radiation, permitting the incident electromagnetic radiation to be reflected upwards back into the photoresist layer. Therefore, the incident path length of electromagnetic radiation from the top surface of the photoresist to the reflective layer can be larger for transparent barrier layers than for opaque layers. An increased incident path length permits greater lateral displacement of the beam. For each incremental increase in lateral displacement of the incident beam, there is a corresponding incremental increase in lateral displacement of the reflected beam.
Diffraction and reflection can occur together, thereby amplifying the defects. FIG. 1 is a drawing of a prior art semiconductor device during conventional photolithography, and depicts problems caused by diffraction and reflection at an interface. The substrate 104 has a layer of photoresist 110 on its surface, thereby forming reflective surface 112 between the layers. A mask 116 has an aperture defined by edges 120 through which electromagnetic radiation can penetrate into photoresist layer 110. Incident electromagnetic radiation 124 is diffracted laterally by edges 120. A portion of the diffracted electromagnetic radiation 124 is reflected from surface 112 laterally and passes upwards through photoresist layer 110, resulting in photolysis of the photoresist layer 110 to form a channel 128 having an uneven and non-parallel sidewall surfaces 132.
FIG. 2 is a drawing of a prior art semiconductor device during conventional photolithography with layers of source-drain oxide and stoichiometric barrier and/or polish-stop layers, and depicts the problems caused by diffraction and reflection. A layer of oxide 105 is deposited on the top of the substrate 104, and a nitride barrier layer 106 is deposited on top of oxide layer 105. A layer of photoresist 110 is deposited on the surface of nitride barrier layer 106. The oxide layer 105, nitride layer 106 and photoresist layer 110 are transparent, so that there is little reflection from surface 112. However, a reflective surface 112 is formed between the layers of oxide 105 and silicon substrate 104. A mask 116 overlays the top of photoresist layer 110, and mask 116 has an aperture defined by edges 120 through which electromagnetic radiation can pass. A monochromatic source of electromagnetic radiation produces a beam of incident electromagnetic radiation 124, which is diffracted laterally at edges 120. A portion of the beam of electromagnetic radiation 124 passes through nitride layer 106 and oxide layer 105 and is reflected by surface 112. The reflected electromagnetic radiation passes upwards through the photoresist layer 110, resulting in photolysis of the photoresist layer 110 to form a channel 128 having an uneven and non-parallel sidewall surfaces 132.
C. Interference Effects and Standing Waves
In addition to diffraction and lateral reflection, reflected and incident electromagnetic radiation can interfere with each other to produce standing waves in the photoresist materials. Standing waves provide higher energy to locations of interference maxima than to other areas. The difference in energies delivered to different locations within the photoresist layer can produce uneven photolysis, which results in a type of defect herein termed "scalloping." Additionally, standing waves can cause non-vertical resist profiles, variations in line width, reflective notching, scumming and alignment inaccuracies. Therefore, standing waves can be great enough to make it difficult to obtain repeatable patterns in the photoresist, particularly at the thicknesses used in source-drain masking
FIG. 3 is a drawing depicting the formation of a standing wave within a layer of photoresist material 110 overlying a substrate 104 having a reflective surface 112 between the substrate and photoresist material. An incident beam of monochromatic electromagnetic radiation 125 strikes reflective surface 112 and is reflected back upwards as a reflected beam 126. As incident beam 125 and reflected beam 126 interact, an interference pattern is generated, resulting in a standing wave 127, having areas of interference maxima 130.
FIG. 4 is a drawing depicting the effect on photolysis of photoresist materials of the formation of a standing wave as in FIG. 3. As in FIG. 3, a semiconductor substrate 104 has a layer of photoresist material 110 deposited thereon in such a fashion as to produce a reflective surface 112 therebetween. As the photoresist material 110 is exposed to the beam of electromagnetic radiation, the standing wave (127 as in FIG. 3) photolyzes the photoresist material 110, resulting in the formation of a channel 128 having an uneven, scalloped edge 132 in channel 128 with large amplitude variations corresponding to interference maxima 130.
The aggregated effects of diffraction, reflection and standing waves can result in uneven and non-parallel sidewall surfaces. The production of these channel defects are herein termed "channel broadening."
To address the problems of channel broadening, several approaches can be used. One approach to minimize the problems of reflection is by increasing the absorption of electromagnetic radiation by the photoresist material. By increasing the absorption of electromagnetic radiation by the photoresist layer, less radiation can reach the underlying interface, and therefore, less reflection of the incident electromagnetic radiation occurs. Increasing absorption of electromagnetic radiation can be accomplished by the use of dye-containing photoresist layers or the use of thicker or multiple layers of photoresist materials.
IV. Anti-Reflective Coatings
In addition to altering the photoresist layers themselves, another approach involves using anti-reflective layers. Anti-reflective layers can absorb some of the incident radiation, diminishing the intensity of reflected radiation, and thereby reduce beam spreading and standing waves. Anti-reflective coatings can have advantages over thicker photoresist layers because thinner photoresist layers provide less opportunity for channel broadening due to misdirected electromagnetic radiation.
An example of a photolithography process involving a conventional anti-reflective coating is shown in FIG. 5. The semiconductor device 500 has a silicon substrate 104 having layers of oxide 105 and nitride 106 deposited thereon. Anti-reflective layer 107 is deposited on top of the nitride layer, and a photoresist layer 110 is deposited on top of anti-reflective layer 107. A mask 116 having an aperture defined by edges 120 overlays photoresist layer 110. A source of monochromatic electromagnetic radiation (not shown) produces an incident beam of radiation 124, which can pass through the aperture in mask 116. Incident beam 124 is diffracted laterally by edges 120. Incident beam 124 passes through photoresist layer 110 causing photolysis. Incident beam 124 does not penetrate the anti-reflective layer 107, the nitride layer 106, or the oxide layer 105.
However, there can be several problems associated with conventional anti-reflective layers. Deposition of anti-reflective layer 107 requires additional steps, materials and time, thereby decreasing manufacturing efficiency an increasing cost. Moreover, separate anti-reflective layers can generate relative mechanical stresses within the underlying semiconductor layers, which can lead to defects in the films, thereby decreasing photolithography resolution. Additionally, inorganic anti-reflective layers can alter the chemical reactions that occur in the photoresist layers, leading to a phenomenon termed "photoresist poisoning." Photoresist poisoning can result in a loss of sensitivity of the photoresist to the electromagnetic radiation, leading to incomplete photolysis, thereby producing a defect termed a "foot." A foot is an area of incompletely photolyzed photoresist material which narrows the channel in the photoresist. Photoresist poisoning is unpredictable and can therefore lead to poorly reproducible semiconductor feature sizes.
A method for decreasing photoresist poisoning includes treating the surface of the anti-reflective layer with an O.sub.2 plasma. However, this process requires additional processing steps and can cause undesired oxidation of semiconductor materials. Thus the problems associated with reflection and standing waves have been inadequately addressed by the currently available methods of providing anti-reflective layers in photolithography.
Thus, in light of the above problems, one object of this invention is the development of methods for providing anti-reflective coatings which combine etch-stop and/or barrier functions for use in photolithography which are inexpensive to apply.
Another object of this invention is to develop methods for carefully controlling the optical properties of anti-reflective barrier and/or polish-stop layers during deposition.
A further object of this invention is to develop methods for providing anti-reflective barrier and/or polish-stop layers which can be easily incorporated into existing manufacturing methods.
Another object of this invention is to develop methods for providing reproducible and diminished reflected electromagnetic radiation defects during the use of monochromatic electromagnetic radiation for photolithography.
An additional object of this invention is to provide methods for photolithography in which small semiconductor device features can be made.